Regulation of mouse hepatic cytochrome P450 2A5 during hepatocellular injury and inflammation
Murine hepatic cytochrome P450 2A5 (CYP2A5) is uniquely induced by a variety of agents that cause hepatotoxicity and hepatitis, pathophysiological conditions that are typically associated with down-regulation of other CYPs. An examination of these biological and chemical inducers suggests that CYP2A5 induction is not directly related to the inducing agents, but is possibly a consequence of liver injury or the associated inflammatory response. We have tested the hypothesis that induction of murine liver CYP2A5 during these conditions occurs as a result of a common stress-stimulus associated with liver injury and hepatitis. The influence of acute inflammation versus hepatocellular injury on CYP2A5 induction was compared in mouse livers. Results showed that interleukin-1[beta] (IL-1[beta]) and IL-6 did not affect the expression of hepatic CYP2A5, whereas lipopolysaccharide (LPS) down-regulated CYP2A5 levels. In contrast, induction of CYP2A5 by pyrazole was associated with hepatocellular injury, particularly to the endoplasmic reticulum (ER) as indicated by co-localization of CYP2A5 to the ER stress protein glucose-regulated protein-78 (GRP78) within damaged pericentral hepatocytes. A further analysis of the role of ER stress as a common inducing condition was carried out. While both pyrazole and phenobarbital induced CYP2A5, only pyrazole increased GRP78 and GRP94 levels in mouse livers ' in vivo' and in primary cultures of mouse hepatocytes. Furthermore, treatment of hepatocytes with various ER stressors revealed that only dithiothreitol (DTTox), which alters cellular redox, resulted in increased CYP2A5 levels. The association between pyrazole-mediated CYP2A5 induction and oxidative stress was further supported by dose- and time-dependent increases in the levels of oxidized protein as determined by protein carbonyl content, and by increased levels of the antioxidant enzyme glutathione S-transferase (GST) Mu. Furthermore, pre-treatment of hepatocytes with antioxidants, N-acetylcysteine and vitamin-E, prevented pyrazole-mediated induction of CYP2A5. Moreover, pro-oxidant treatment of hepatocytes with the redox-cycling agent menadione increased CYP2A5 expression. These studies indicate that CYP2A5 induction is associated with oxidative stress. In conclusion, oxidative stress represents a common stimulus that leads to the induction of murine hepatic CYP2A5 during liver injury, and suggests that CYP2A5 may play a cytoprotective role during oxidative stress.